Add websocket session support to kaya-session

- kaya-core: WebSocket ABC gains session attribute and accept(headers=...)
- kaya-core: AsgiWebSocket injects headers into websocket.accept message
- kaya-rsgi: RsgiWebSocket accepts headers param (ignored — Granian's
  accept() takes no args)
- kaya-session: SessionWebSocket wrapper exposes ws.session and injects
  Set-Cookie on accept()
- kaya-session: SessionMixin registers before/after websocket hooks;
  session loaded at connect, persisted on close if modified
- 10 new WV session tests covering read, persist, handshake cookie,
  regenerate, invalidate, isolation
- Example and README updated

example/session.py

@@ -1,4 +1,4 @@
-from kaya.core import HttpContext, KayaApp
+from kaya.core import HttpContext, KayaApp, WebSocket
 from kaya.session import InMemorySessionStore, SessionMixin
 
 app = KayaApp(mixins=[SessionMixin(InMemorySessionStore())])
@@ -35,11 +35,7 @@ async def echo(ws: WebSocket) -> None:
 
 @app.websocket('/ws/visits')
 async def ws_visits(ws: WebSocket) -> None:
-    # WebSocket handlers can read the existing session. Most ASGI servers
-    # (including Granian and Daphne) do not forward the `headers` field of the
-    # `websocket.accept` message into the HTTP 101 response, so a new session
-    # cookie cannot be set during the handshake. Use the HTTP `/` endpoint to
-    # set or refresh the session cookie before connecting here.
+    visits = ws.session.get('visits', 0) + 1
+    ws.session['visits'] = visits
     await ws.accept()
-    visits = ws.session.get('visits', 0)
     await ws.send_text(f'visits: {visits}')